Electronic consumer products are pushing both the bounds of portability and computation complexity, in certain cases, simultaneously. Today mobility implies that the product has attributes such as the capability of being wireless. In addition, since video is playing a larger role in our lives every day, the need for low power computation techniques and high performance for video applications for both mobile and desktop systems is required.
An oscillator block provides the ability to regulate the flow of computation data within a VLSI (Very Large Scale Integration). For instance, the on chip clock frequency of a high-end microprocessor is expected to reach 10 GHz before the end of this decade. In addition, the power dissipation for the microprocessor is expected to be about 200 W, where the clock network will consume almost half of this power or 100 W. Thus, for this microprocessor, the higher frequencies and larger power dissipation values indicate a need to have clock circuits that can easily generate a 10 GHz signal and should be able to reduce the power dissipation of the clock network. The clock network of these VLSI chips typically contains large values of capacitance that need to be driven.
Handheld units are driving the desire for the ubiquitous need for wireless. Due to the limited energy storage ability of batteries, energy conservation is paramount for longer play and talk times. These units contain a mixture of analog and digital components. Analog circuits are used in the radio frequency (RF) sections of the wireless blocks that typically contain some form of a clock oscillator. The digital circuits will require a lower power technique of distributing the clock signal within the chip. By minimizing the power dissipation of the clock circuits and networks of the wireless units, the time between charging the batteries of the portable units can be extended.
Some of the basic circuit blocks to help achieve the ability for mobility, low power, and high computation require the necessity of a clock oscillator block. Tank circuits have been used to generate oscillatory clock signals. These circuits use LC (inductor-capacitor) elements to form the tank circuit.
For example, U.S. Pat. No. 5,396,195 issued Mar. 7, 1995 to Gabara depicts a basic LC tank circuit in an MOS technology. Several examples are given where a cross-coupled MOS circuit drives the tank circuit. The oscillations generated by the MOS LC tank circuit fabricated in a 0.9 μm CMOS technology operated with a supply voltage of 3.3V. The power dissipation was reduced by a factor of a 10× when a capacitive load was driven using an LC tank circuit as compared to being driven using conventional digital techniques. This circuit has been used in a multitude of applications ranging from wireless to on-chip clock generation modules. Many of the inductors used in this type of tank circuit have the form of the horizontal planar inductor as illustrated in FIG. 1a and FIG. 1b. These type of inductors typically require a large amount of area to form the inductor.
The calculations of the values of these type of inductors is provided in a published paper, “Simple Accurate Expressions for Planar Spiral Inductances”, IEEE J. Solid-State Circuits, Vol. 34, No. 10, October 1999, by Mohan et al., hereafter referred to as the “Mohan” reference.
In addition, the Q or quality factor of these inductors that are fabricated in CMOS are typically low. The quality factor or Q is a primary parameter in the evaluation of tank circuits.
                    Q        =                  2          ⁢          π          ⁢                                    Maximum              ⁢                                                          ⁢              energy              ⁢                                                          ⁢              stored              ⁢                                                          ⁢              in              ⁢                                                          ⁢              tank              ⁢                                                          ⁢              circuit                                      Energy              ⁢                                                          ⁢              dissipated              ⁢                                                          ⁢              per              ⁢                                                          ⁢              cycle                                                          (        1        )            The Q indicates the amount of energy dissipated by the tank circuit to maintain oscillations. The tank circuit is more energy efficient as the value of the Q term increases which indicates that the energy dissipated in the tank circuit decreases. One way to decrease the dissipation is to reduce the parasitic resistance of the inductor.
Another method to increase the Q for designs above 1 GHz is to reduce the induced eddy current in the conductor of the inductor. As pointed out by Niknejad and Meyer, IEEE Trans. Microwave Theory Tech., Vol. 49, No. 1, January 2001, the eddy current loss within the metallic region of the planar inductors is a dominant loss above 1 GHz.
U.S. Pat. No. 6,759,937 issued Jul. 6, 2004 to Kyriazidou suggests a balanced vertical multi-layer planar inductor to reduce the area and improve the symmetry of the inductor. A vertical planar inductor is very similar to a helix. This helix uses a square coil instead of a circular one. A square helix structure is illustrated in FIG. 1c. This structure offered the benefit of using less real estate and higher Q for a given value of inductance. Kyriazidou achieves this in part by decreasing the resistance of the coil. Their approach is to shunt sections of a lower metal layer winding to sections of a higher metal layer winding by using multiple vias.
U.S. Pat. No. 5,831,331 issued Nov. 3, 1998 to Lee proposed a helix structure to form a vertical multi-layer planar inductor that uses shielding to increase the inductance. A shield formed in the substrate stops the flow of eddy currents in the substrate. This structure also offered the benefit of using less area for a given value of inductance. In addition, Lee desired to decrease the resistance of a coil in a lower metal layer by electrically connecting the lower layer coil to an upper layer coil formed in a higher metal layer. There is a drawback to this reduction of the resistance. As described by Lee, a single via is used to create this electrical connection. Because only one end of the upper layer coil is DC connected to the lower coil (by this single via), the desire to reduce the resistance of the lower coil in not effective since current entering the higher metal level would not have a return path back to the lower coil. This is in stark contrast to the approach of Kyriazidou since Kyriazidou does provide multiple current return paths from sections of the upper metal layer to sections of the lower layer and achieves the goal of reducing the resistance of the coil. Thus, Lee's approach to reducing the resistance of a coil does not achieve its goal.
U.S. Pat. No. 6,480,086 issued Nov. 12, 2002 to Kluge et. al., describes a vertical multi-layer planar inductor to increase the inductance for a given area usage. Kluge uses a helix to create the inductance. In addition, a transformer is described where the second coil is closely spaced to the first coil to achieve a magnetic coupling between the two coils. Kluge indicates the use of multiple vias to reduce the series resistance. However, this resistance reduction is directed to the via connection itself.
Because real estate is expensive, reducing the area used to from the inductors would be beneficial. In addition, it is desirable to address power dissipation reduction issues in the design of inductors. The first is to decrease the parasitic resistance of a coil so that losses are minimized. Next, it is desirable to decrease the eddy current loss within the metallic inductor. Doing so offers an increase in the Q of the tank circuit and provides the added benefit of reducing the power dissipation of the tank circuit. This application will address these and other issues necessary to help achieve these goals.